The firing of a firearm is typically controlled by a trigger mechanism. The trigger mechanism includes a trigger that, when pulled, releases spring-loaded components that initiate the firing sequence.
The characteristics of the pull of a trigger mechanism (also known as the “dynamics” of the trigger) greatly impact the accuracy of the firearm in different situations. The characteristics of the trigger pull are sometimes described by the takeup, the break, and the overtravel. The takeup is the amount of movement of the trigger until it comes to a point slightly before the trigger releases. The break involves the movement of the trigger from the point slightly before the release to the point of release. The overtravel is the distance that the trigger moves after the sear releases.
Trigger mechanisms are sometimes classified by a number of stages, such as including single stage triggers and two-stage triggers. A single stage trigger has no discernible movement (takeup) before the break, while a two-stage trigger has a noticeable takeup (the first stage), followed by a distinct increase in the force required to pull the trigger (the second stage) before the break. Each of these types of trigger mechanism has its benefits. The single stage trigger is quick and simple, and can be particularly useful when firing multiple shots in rapid succession. The two-stage trigger can be highly accurate by minimizing movement at the moment of the break.
Although the trigger mechanism can be replaced in some firearms, most firearms include only a single trigger mechanism, and therefore the trigger mechanism having the desired pull characteristics must be selected and installed into the firearm before the firearm can be used.